An Optical Transport Network (OTN) is defined by ITU-T as a set of optical elements that are connected by optical links and that are capable of providing transporting, multiplexing, switching, management, and supervision functionality, and survivability of optical channels carrying client signals.
An OTN uses digital wrapper technology that provides a method for encapsulating data in an optical data unit. Generally, digital wrapping involves grouping a number of existing optical data units together into one entity that can be more efficiently managed with a small allocation of overhead and forward error correction (FEC) bytes. There are three levels of hierarchy for the encapsulation: the optical channel payload unit (OPUk) for encapsulation of client data; the optical channel data unit (ODUk) for encapsulating the OPUk data; and the optical channel transport unit (OTUk) for encapsulating the ODUk data.
From the perspective of a node in the optical network, the functions required to support the end-to-end transport of the client signals may include:                (1) Select an ODUk container that can transport the client signal.        (2) Determine if the network can support an end-to-end ODUk connection of the bit rate identified in step (1).        (3) If the network can support an end-to-end ODUk connection, the single ODU container occupies a single wavelength at every hop along the route from the source node to the destination node. Select and configure the correct ODU mappers/demappers at the ingress and egress nodes. The service setup has succeeded.        (4) If the network cannot support an end-to-end ODUk connection, the available network bandwidth is fragmented and the required bandwidth is not available within a single wavelength along every hop, but is distributed across multiple wavelengths. Determine if the network can route L independent ODUj connections end-to-end, where j<k, and L depends on the client signal rate, and the bit rate of the ODUj container. Find the largest j<k for which this step can succeed. If no combination of L ODUj containers can be found, the service setup fails.        (5) Instantiate a Virtual Concatenation Group (VCG) at the head and tail-end nodes, and configure the hardware appropriately, where the VCG members are ODU connections which are routed independently and, thus, encounter different latencies in their transit through the network. The service setup has succeeded.        
The head-end node distributes the data from the client signal in a round-robin fashion among the VCG members. The ODU containers for all of the VCG members start out being frame (and frequency) aligned at the head-end node. The frames may arrive skewed at the receiver because of possible differing path latencies. The tail-end node performs a “deskew” operation to align the various VCG members, and then reconstructs the client signal by sequencing the data from the VCG members in a round-robin fashion.
The VCG approach creates operational difficulties for network operators. For example, network operators lose the 1:1 correspondence between a service and the ODU instance that is transporting the tributary signal supplied by the customer edge device. In the VCG scenario, some services are supported by multiple ODU server layers. Also, the VCG approach may create a need to monitor multiple ODU connections for each service. This would mean monitoring each of the ODU connections to gather alarms and performance information. Further, the head-end node may need to support the independent restoration/reroute of each of these ODU connections (upon the detection of a failure). The protection/restoration of the individual VCG members needs to be coordinated to ensure that the end-to-end skew between the VCG members is within the range that the tail-end can handle. Moreover, even when a service was initially setup via mapping to a single ODU container, the restoration that occurs following a network resource failure (e.g. link/node) may make use of a VCG containing multiple lower bit rate ODU containers. More specifically, each service could be set up with or without the use of VCG. This aspect makes it difficult for the network operators to troubleshoot and fix network issues that occur.